ceph: messenger: rework prepare_connect_authorizer()

Change prepare_connect_authorizer() so it returns without dropping
the connection mutex if the connection has no get_authorizer method.

Use the symbolic CEPH_AUTH_UNKNOWN instead of 0 when assigning
authorization protocols.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>